This application claims priority based on French Patent Application No. 01/05426, filed on Apr. 23, 2001.
The invention relates to the domain of static sealing, notably the elastic metal gasket that must provide a seal with a preload less than that necessary to ensure the efficiency of a gasket with a spring core.
In view of their insensitivity to corrosive fluid, their good behavior at high and low temperatures and their good resistance over time, metal gaskets are used for a variety of applications, among which can be cited, in a non-limiting manner, the chemical, petroleum and nuclear industries, as well as the automotive and aerospace sectors. The quality of the seal provided by a gasket depends notably on the specific pressure developed between the contact surfaces of the gasket and the flanges of the assembly in which the gasket is placed. During the initial tightening of the assembly, the specific pressure must be sufficient to allow the adaptation of the gasket to the surface irregularities of the flanges. It is thus clear that the specific pressure of the tightening must be relatively high, in any event, greater than the pressure of the fluid found inside the volume delimited by the gasket and the surfaces of the tightening flanges.
Furthermore, in many applications, the preload must remain low. This is notably the case when the assembly is not very accessible, thus making the handling of tightening tools difficult such as in the nuclear industry and when the assemblies made of high-characteristic materials must be lightened and cannot withstand significant preload, as in the aerospace and space industries. To achieve such a result, metal o-ring gaskets have been developed. Their elastic element is an open tube, preferably sheathed in a thin layer of ductile material. Each contact surface has a part that projects in the axis of reaction of the gasket with respect to the surfaces of the tightening flange. This reduces preload by a factor of 2 on this type of gasket. The US patent, U.S. Pat. No. 3,188,100 can be cited to this effect. The principles and disadvantages of the operation of the gasket described in the U.S. Pat. No. 3,188,100 patent are explained in reference to FIGS. 1 and 2.
In reference to FIG. 1, showing a cross-section of the gasket described in the American patent, it is observed that the gasket has a ring shape with its section forming a revolution. On the other hand, the latter is open by an opening 2 placed on the inside of the gasket. It is composed of two parts which are a metal core 3 around which is placed a softer external layer 4. The assembly is machined or deformed on the parts that will be in contact with the parts to seal so as to form two projecting parts 5 projecting over a flat surface 6 on either side of the gasket. These two projecting parts 5 are thus designed to come in contact with the two surfaces that the gasket will be in contact with and to be partially flattened against them.
The choice of a general open tube structure does not simultaneously satisfy the criteria of low stiffness of the whole and high leaktightness despite the presence of the projecting parts 5. Indeed, the open tube structure leads to an asymmetrical deformation of the gasket section during flattening and, as a result, to a deterioration of the seal generated by the displacement of the projecting parts 5 as shown in FIG. 2.
On this figure, points C1 and C2 indicate the two contact locations of a projecting part respectively before and after flattening. Gasket 1 was represented in contact with two surfaces that delimit the two spaces to be sealed and which come together when flattened. A displacement of xcex94a of location C1 with respect to location C2 of the contact point occurs. It can even be observed that when gasket 1 is flattened, the extremities 7 of the open structure formed by gasket 1 come together slightly and shift towards the outside. In other words, if the initial position of the projecting parts, identified as 5 on FIG. 1, is found in the center of the geometric structure of the gasket, as point C1, point C2 representing the positioning of the projecting parts, after gasket 1 is flattened, this position is no longer in the center but on one side. The shift of the axis of the section of the gasket is thus prejudicial to achieving the desired seal. In such a gasket, the best sealing level possible is achieved during flattening and not at the end of the flattening operation as is sought by the users. It is even possible to note a deterioration of the seal during flattening.
The French patents FR-2 557 662 and FR-2 636 115 each describe a type of metal o-ring gasket whose core is composed of a helical spring and which has two projecting parts machined on an outside sheath. The choice of a helical spring to form the central core of the gasket does not enable the preload to be decreased below a certain threshold since there is a very high risk of flipping of the turns of the spring and consequently a random reaction of the spring on the sealing surface.
The aim of the invention is to remedy this inconvenience by proposing another type of elastic metal gasket with projecting parts.
To this effect, the main object of the invention is an elastic metal gasket including an elastic metal core and an external metal sheath in which the elastic metal core is encased and which, when the gasket is idle, presents an open circular type section, defining a center line, that is a line passing through the geometric center of the idle section, the opening of the general structure of the gasket being found between two opposite contact surfaces each having a projecting part, whose summit is designed to come in sealed contact with an object under a determined preload.
According to the invention, when idle, the projecting parts are slightly offset on the same side with respect to the center line such that they come closer to this center line under the effect of preload.
Preferably, the projecting parts are equidistant from the opening of the structure of the gasket.
In the preferential execution of the seal according to the invention, the two contact surfaces are each perpendicular to an axis of symmetry of its related projecting part whose axis of symmetry passes through the center of the circle defined by the circular section of the gasket.
In this case of the preferential execution of the seal according to the invention, when the gasket is idle, each contact surface is inclined at a determined angle with respect to the positioning plane defined by the summit of the projecting part, when it is triangular in shape, and the extremity of the surface found closest to the opening.
In a great many executions of the seal according to the invention, the gasket is generally ring-shaped. In other executions of the seal according to the invention the gasket may be elliptical, rectangular, generally rectangular with rounded corners, triangular or another of the representative shapes shown in FIGS. 8-10.
Preferably, the height of the projecting parts falls between 0.05 and 0.15 mm. For an approximately triangular section the height is measured between the summit of the projecting part and the contact surface of the external metal sheath. In other executions of the seal according to the invention the projecting part may be elliptical, rectangular, generally rectangular with rounded corners, triangular or another of the representative shapes show in FIGS. 11-14. For an approximately trapezoidal section, rounded section or other shaped section, the height is measured perpendicular to the contact surface of the external metal sheath at the summit of the projecting part.